Molecular biology and pharmaceutical drug development now make intensive use of nucleic acid analysis (Friedrich, G. A. Moving beyond the genome projects, Nature Biotechnology 14, 1234 (1996)). The most challenging areas are whole genome sequencing, single nucleotide polymorphism detection, screening and gene expression monitoring. Currently, up to hundreds of thousands of samples are handled in single DNA sequencing projects (Venter, J. C., H. O Smith, L. Hood, A new strategy for genome sequencing, Nature 381, 364 (1996)). This capacity is limited by the available technology. Projects like the “human genome project” (gene mapping and DNA sequencing) and identifying all polymorphisms in expressed genes involved in common diseases imply the sequencing of millions of DNA samples.
With most of the current DNA sequencing technologies, it is simply not possible to decrease indefinitely the time required to process a single sample. A way of increasing throughput is to perform many processes in parallel. The introduction of robotic sample preparation and delivery, 96 and 384 well plates, high density gridding machines (Maier, E., S. Meierewer, A. R. Ahmadi, J. Curtis, H. Lehrach, Application of robotic technology to automated sequence fingerprint analysis by oligonucleotide hybridization, Journal Of Biotechnology 35,191 (1994)) and recently the development of high density oligonucleotide arrays (Chee, M., R. Yang, E. Hubbell, A. Berno, X. C. Huang, D. Stern, J. Winkler, D. J. Lockhart, M. S. Morris, and S. P. A. Fodor, Accessing genetic information with high-density DNA arrays, Science 274(5287):610-614, (1996)) are starting to bring answers to the demand in ever higher throughput. Such technologies allow up to 50,000-100,000 samples at a time to be processed within days and even hours (Maier, E., Robotic technology in library screening, Laboratory Robotics and Automation 7, 123 (1995)).
In most known methods for performing nucleic acid analysis, it is necessary to first extract the nucleic acids of interest (e.g., genomic or mitochondrial DNA or messenger RNA (mRNA)) from an organism. Then it is necessary to isolate the nucleic acids of interest from the mixture of all nucleic acids and usually, to amplify these nucleic acids to obtain quantities suitable for their characterisation and/or detection. Isolating the nucleic fragments has been considered necessary even when one is interested in a representative but random set of all of the different nucleic acids, for instance, a representative set of all the mRNAs present in a cell or of all the fragments obtained after genomic DNA has been cut randomly into small pieces.
Several methods can be used to amplify DNA with biological means and are well known by those skilled in the art. Generally, the fragments of DNA are first inserted into vectors with the use of restriction enzymes and DNA ligases. A vector containing a fragment of interest can then be introduced into a biological host and amplified by means of well established protocols. Usually hosts are randomly spread over a growth medium (e.g. agar plates). They can then replicate to provide colonies that originated from individual host cells.
Up to millions of simultaneous amplification of cloned DNA fragments can be carried out simultaneously in such hosts. The density of colonies is of the order of 1 colony/mm2. In order to obtain DNA from such colonies one option is to transfer the colonies to a membrane, and then to immobilise the DNA from within the biological hosts directly to the membrane (Grunstein, M. and D. S. Hogness, Colony Hybridization: A method for the isolation of cloned DNAs that contain a specific gene, Proceedings of the National Academy of Science, USA, 72:3961 (1975)). With these options however, the amount of transferred DNA is limited and often insufficient for non-radioactive detection.
Another option is to transfer by sterile technique individually each colony into a container (e.g., 96 well plates) where further host cell replication can occur so that more DNA can be obtained from the colonies. Amplified nucleic acids can be recovered from the host cells with an appropriate purification process. However such a procedure is generally time and labour consuming, and difficult to automate.
The revolutionary technique of DNA amplification using the polymerase chain reaction (PCR) was proposed in 1985 by Mullis et al. (Saiki, R., S. Scharf, F. Faloona, K. Mullis, G. Horn, H. Erlich and N. Arnheim, Science 230, 1350-1354 (1985) and is now well known by those skilled in the art. In this amplification process, a DNA fragment of interest can be amplified using two short (typically about 20 base long) oligonucleotides that flank a region to be amplified, and that are usually referred to as “primers”. Amplification occurs during the PCR cycling, which includes a step during which double stranded DNA molecules are denatured (typically a reaction mix is heated, e.g. to 95° C. in order to separate double stranded DNA molecules into two single stranded fragments), an annealing step (where the reaction mix is brought to e.g., 45° C. in order to allow the primers to anneal to the single stranded templates) and an elongation step (DNA complementary to the single stranded fragment is synthesised via sequential nucleotide incorporation at the ends of the primers with the DNA polymerase enzyme).
The above procedure is usually performed in solution, whereby neither the primers nor a template are linked to any solid matrix.
More recently, however, it has been proposed to use one primer grafted to a surface in conjunction with free primers in solution in order to simultaneously amplify and graft a PCR product onto the surface (Oroskar, A. A., S. E. Rasmussen, H. N. Rasmussen, S. R. Rasmussen, B. M. Sullivan, and A. Johansson, Detection of immobilised amplicons by ELISA-like techniques, Clinical Chemistry 42:1547 (1996)). (The term “graft” is used herein to indicate that a moiety becomes attached to a surface and remains there unless and until it is desired to remove it.) The amplification is generally performed in containers (e.g., in 96 well format plates) in such a way that each container contains the PCR product(s) of one reaction. With such methods, some of the peR product become grafted to a surface of the container having primers therein which has been in contact with the reactant during the PCR cycling. The grafting to the surface simplifies subsequent assays and allows efficient automation.
Arraying of DNA samples is more classically performed on membranes (e.g., nylon or nitro-cellulose membranes). With the use of suitable robotics (e.g., Q-bot™, Genetix ltd, Dorset BH23 3TG UK) it is possible to reach a density of up to 10 samples/mm2. Here, the DNA is covalently linked to a membrane by physicochemical means (e.g., UV irradiation). These technologies allow the arraying of large DNA molecules (e.g. molecules over 100 nucleotides long) as well as smaller DNA molecules. Thus both templates and probes can be arrayed.
New approaches based on pre-arrayed glass slides (arrays of reactive areas obtained by ink-jet technology (Blanchard, A. P. and L. Hood, Oligonucleotide array synthesis using ink jets, Microbial and Comparative Genomics, 1:225 (199)) or arrays of reactive polyacrylamide gels (Yershov, G. et al., DNA analysis and diagnostics on oligonucleotide microchips, Proceedings of the National Academy of Science, USA, 93:4913-4918 (1996)) allow the arraying of up to 100 samples/mm2. With these technologies, only probe (oligonucleotide) grafting has been reported. Reported number of samples/mm2 are still fairly low (25 to 64).
Higher sample densities are achievable by the use of DNA chips, which can be arrays of oligonucleotides covalently bound to a surface and can be obtained with the use of micro-lithographic techniques (Fodor, S. P. A. et al., Light directed, spatially addressable parallel chemical synthesis, Science 251:767 (1991)). Currently, chips with 625 probes/mm2 are used in applications for molecular biology (Lockhart, D. J. et al., Expression monitoring by hybridisation to high-density oligonucleotide arrays, Nature Biotechnology 14:1675 (1996)). Probe densities of up to 250 000 samples/cm2 are claimed to be achievable (Chee, M. et al., Accessing genetic information with high-density DNA arrays, Science 274:610 (1996)). Currently, up to 132000 different oligonucleotides can be arrayed on a single chips of approximately 2.5 cm2. Presently, these chips are manufactured by direct solid phase oligonucleotide synthesis with the 3′OH end of the oligo attached to the surface. Thus these chips have been used to provide oligonucleotide probes which cannot act as primers in a DNA polymerase-mediated elongation step.
When PCR products are linked to the vessel in which PCR amplification takes place, this can be considered as a direct arraying process. The density of the resultant array of PCR products is then limited by the available vessel. Currently available vessels are only in 96 well microtiter plate format. These allow only around ˜0.02 samples of PCR products/mm2 of surface to be obtained.
Using the commercially available Nucleolink™ system obtainable from Nunc A/S (Roskilde, Denmark) it is possible to achieve simultaneous amplification and arraying of samples in containers on the surface of which oligonucleotide primers have been grafted. However, in this case the density of the array of samples is fixed by the size of the vessel. Presently a density of 0.02 samples/mm2 is achievable for the 96 well plate format. Increasing this density is difficult. This is apparent since, for instance, the availability of 384 well plates (0.08 samples/mm2) suitable for PCR has been delayed due to technical problems (e.g. heat transfer and capillary effects during filling). It is thus unlikely that orders of magnitude improvements in the density of samples arrayed with this approach can be achieved in the foreseeable future.